1. Field of the Invention
The present invention relates in general to a method and apparatus for securing pressure-containing threaded tubular connections in remote or hazardous locations. More particularly, the invention relates to a method and apparatus for using electrical heating to remotely pretension pressure containing threaded tubular connections.
2. Description of the Related Art
Tubular connections for containing high pressure are widely used by industry, and a very large number of different types have been developed for specific needs. Several types based upon threaded connections are used frequently in oilfield applications. One very common type of connection used for pipes has tapered threads and relies upon interference between the male and female threads when the connection halves are screwed together. The National Pipe Thread (NPT) commonly used in the United States belongs to this first type of connection. This type of connection requires high make up and break out torques and liberal lubrication for even small sizes.
A second type of threaded connection utilizes the interference fit of a transverse sealing shoulder on each side of the connection where the annular shoulders are forced together as the threads are made up. This second type of connection can use either integral threads so that the sealing shoulders must be rotated relative to each other or a nut on one side can be used to avoid relative rotation of the sealing shoulders. Standard American Petroleum Institute (API) drill pipe threads are a typical example of a relatively rotating seal of this second type of connection. For large sizes, very high make up torques and ample lubrication are required. This type of connection can be modified to use an annular face seal in a groove in one of the comating sealing shoulders either as the only seal or as a secondary seal. Certain types of seals such as elastomers or metal C-rings or metal O-rings do not require high compression loads to achieve sealing, but high interfacial preloads may be needed in order to resist high tensile or bending loadings transferred across the connection. Another type of modification for this type of seal uses an intermediate annular seal washer compressed between the two transverse sealing faces.
A third type of connection is similar to the second, but uses the interference fit of comating conical faces to achieve sealing. Again, the connection can have either relative rotation or no relative rotation. This type of connection is frequently used in oilfield tubing and casing. The torques to achieve sealing are somewhat reduced, but the hoop stresses adjacent the comating conical surfaces must be controlled by limiting the interference fit. Again, the torques are only somewhat reduced and good lubrication is still required.
A fourth type of screwed connection uses a seal ring in an annular groove on a cylindrical surface of a first connection half to achieve radial sealing between the grooved part and a comating cylindrical surface on the second connection half. The seal can be elastomeric, such as an O-ring or a C-ring, or the seal can be a metal O-ring or some other metal-to-metal seal. This type of connection does not require much lubrication or high make up torques unless high loadings must be resisted across the connection. The seal for this type of connection can be either rotating or nonrotating relative to its comating surface.
Another type of connection uses flanges having multiple bolts or studs and nuts in a bolt circle pattern to pull the two sides of the connection together and achieve sealing. API or ANSI flanges are examples of this type of connection. Either annular face seals with or without intermediate gaskets or annular wedging interference fit seals such as conical faced ring gaskets are typically used. Lubrication is not as severe a problem as for the first, second, and third connection types. However, even though the torques required to make up or break out the fasteners are less than those for a single thread pair joint, they can still be substantial. Furthermore, unpredictable variations in friction make obtaining uniform bolting preloads unlikely, thereby impairing joint reliability.
The basic problem with all existing types of connections when used to transmit high axial and bending loadings or to seal high pressures is the need to accurately control the make up torque and friction of the threads so that a predictable value of joint preload can be obtained. This is essential, even for cases where the seal does not require high preload, so that the connection can resist bending moments and the threads of any connections are not subject to large magnitude load cycling with attendant metal fatigue. Unfortunately, such predictability is very hard to obtain, and the predictability becomes much less when the connections are remotely made in a difficult environment, such as subsea. When the connection must be repeatedly made up and broken out remotely, joint torque predictability and, hence, connection and seal reliability become very problematic. For such situations, thread galling, seal damage, misalignment, and the need for very high torques further complicate the problem.
What is urgently needed is a new type of high pressure, high load connection which can be made up and broken out repeatedly without significant risk of failure and without the need for very high torques. Avoidance of seal damage, thread damage, alignment problems, reduced sensitivity to lubrication problems, and general improvements in reliability are strongly needed. Improvements in connection behavior predictability will result if these needs are met. These requirements become particularly critical for remote connections where general robustness of the connection is essential and the connection must be adaptable to widely varying and difficult installation conditions with only limited informational feedback to the installer.